Method for producing ammonia on the basis of a nitrogen-hydrogen mixture from natural gas

ABSTRACT

A method for producing ammonia from natural gas, fed to an autothermic reformer with an O 2  rich gas. Crude synthesis gas is produced at temperatures of 900 to 1200° C., pressures of 40 to 100 bar and in the presence of a cracking catalyst. The crude synthesis gas is led through a catalytic conversion system to convert CO to H 2 , thereby obtaining a conversion synthesis gas with a H 2  content of at least 55 vol.-% and a CO content of not more than 8 vol.-%. The conversion synthesis gas is subjected to a gas purification to remove CO 2 , CO and CH 4 , thereby producing an N 2 -H 2  mixture that is subjected to a catalytic ammonia synthesis. The ammonia produced can at least be partially converted to urea by reacting it with CO 2 .

[0001] The invention relates to a process for the catalytic production of ammonia from a nitrogen/hydrogen mixture.

[0002] The production of an ammonia synthesis gas is known from German patent 2007441 wherein a raw gas is produced by gasifying hydrocarbons, this raw gas being desulfurized, converted, freed from CO₂ and finally subjected to a liquid nitrogen wash to remove residual impurities. A similar process is described in EP patent 0307983 wherein converted synthesis gas is submitted to a liquid nitrogen wash upstream of the ammonium synthesis. Details of the catalytic production of ammonia are to be found in Ullmann's Encyclopedia of Industrial Chemistry, 5^(th) edition, volume A2, pages 143-215; the production of urea is described there in volume A27, pages 333-350. A process for the combined production of ammonia and urea is outlined in EP-A-0905 127.

[0003] The objective of the invention is to operate the ammonia synthesis process at a low cost and to provide a method which is well suited for large plants. In accordance with the invention, this is achieved by routing natural gas—jointly with O₂-rich gas—to an autothermal reformer where, at temperatures in the range of 900-1200° C., a pressure of 40 to 100 bar and in the presence of a reforming catalyst, raw synthesis gas is produced which, on a dry basis, exhibits an H₂ content of 55-75% (vol.), a CO content of 15-30% (vol.) and a CO₂ content of 5-30% (vol.), at an H₂:CO volume ratio of 1.6-4, the raw synthesis gas being extracted from the autothermal reformer, cooled, routed through a catalytic converter to convert CO₂ into H₂ and extracting converted synthesis gas with an H₂ content, on a dry basis, of not less than 55% (vol.) and a CO₂ content of not more than 8% (vol.), submitting the converted synthesis gas to a multistage gas wash for removing CO₂, CO and CH₄, producing an N₂-H₂ mixture which is passed to an ammonia synthesis unit for the catalytic production of ammonia.

[0004] It is important for the process to do without a plant for steam reforming for producing the raw synthesis gas. The autothermal reformer can be operated at relatively high pressures in the range of 30-100 bar, preferably 40-80 bar. Downstream of the reformer, this high pressure can virtually be maintained so that prior to entering the ammonia synthesis unit, the gas only has to be slightly compressed. This is much less cost intensive than conventional methods including steam reforming which only allow relatively low pressures. Autothermal reformers have another advantage over steam reforming in that they supply gas at an appropriate H₂/CO₂ ratio so that, after conversion with the CO₂ accruing in the gas wash, the entire NH₃ produced can be converted to urea.

[0005] A favorable, advanced embodiment consists in converting the ammonia produced in the ammonia synthesis unit at least partly to urea by conversion with CO₂. It is of advantage in this context that CO₂ is removed from the converted synthesis gas in at least one gas washing stage and used for producing urea. One of several possibilities is the combined process described in EP-A-0905 127. Under normal circumstances, the CO₂ obtained in the gas washing stage is absolutely sufficient to meet the CO₂ demand of the urea synthesis, in contrast to conventional processes.

[0006] The CO₂ is preferably removed from the converted gas mixture by means of a physical wash process e.g. operated with methanol at temperatures from −20 to −70° C. In this process, only a relatively small amount of energy, including compression energy, is consumed. At the same time, at least half of the CO₂ can be recovered in the regeneration of the washing liquid at pressures e.g. in the range of 2-8 bar so that compression energy is saved in the subsequent application of CO₂ for the production of urea.

[0007] Conveniently, the O₂-rich gas supplied to the autothermal reformer should have an O₂ content of not less than 70% (vol.), preferably at least 90% (vol.). In this way, the content of impurities in the raw synthesis gas is reduced and the gas washing stage can be designed smaller.

[0008] Embodiment options of the process are explained with the aid of the drawing.

[0009]FIG. 1 shows a process flow diagram.

[0010]FIG. 2 shows a flow diagram of an alternative process

[0011] According to FIG. 1, the pretreatment unit (40) is fed with natural gas through line (1) and water vapor through line (1 a) in order to carry out the desulfurization, heating and removal of the CO₂₊ components according to the state of the art. The pretreatment unit (40) is also fed with methane-containing gas through line (42). A mixture primarily consisting of methane and water vapor flows through line (43) to the burner (2) of an autothermal reformer (3) and simultaneously, through line (4), O₂-rich gas with an O₂ content of usually 70% (vol.), but preferably not less than 95% (vol.), is supplied. The O₂-rich gas originates from an air separation plant (5). The reformer (3) contains a fixed bed (3 a) of a state-of-the-art granular reforming catalyst on nickel basis, for example. In the reactor, a pressure in the range of 30-100 bar, preferably 40-80 bar, prevails whilst the temperatures are in the range of 900-1200° C. The raw synthesis gas extracted through line (7) exhibits an H₂ content of 55-75% (vol.), a CO content of 15-30% (vol.), a CO₂ content of 5-30% (vol.) and an H₂:CO volume ratio of 1.6-4. After cooling in heat exchanger (8), the raw synthesis gas is fed through line (9) to a shift conversion stage (10) which may also consist of several reactors. A temperature range of 150-500° C. and preferably 280-450° C. is applied using state-of-the-art catalysts on iron basis, for instance. CO+H₂O are catalytically converted to CO₂+H₂. The gas in line (11) preferably has an H₂:CO₂ volume ratio of 2.5-3 (dry basis).

[0012] The converted synthesis gas withdrawn through line (11) has an H₂ content, on a dry basis, of not less than 55% (vol.) and preferably at least 65% (vol.) as well as a CO₂ content of not more than 8% (vol.). This gas is initially passed through an indirect cooling stage (12), being subsequently fed through line (13) to a gas washing unit (14) in order to remove CO₂ in particular. This can be effected, for example, by a physical wash with methanol at temperatures in the range of about −70 to −20° C., options being, for example, a methyl diethyl amine wash or the Selexol wash. Spent CO₂-containing washing solution is withdrawn through line (16) and routed to a regenerator (17) in order to remove the CO₂ from the wash solution. Regenerated wash solution is returned to the gas wash (14) through line (18). The accruing CO₂ is excellently suited for being fed to a urea synthesis unit (21) through line (20).

[0013] Partly purified synthesis gas is withdrawn from the gas wash (14) through line (22) and treated in a second wash unit (23) where liquid nitrogen acts as the washing liquid. The nitrogen needed to this end comes from the air separation unit (5), being supplied through line (6). Details of the liquid nitrogen wash for producing NH₃ synthesis gas are to be found in EP patent 0307983 as mentioned above. Typically, the washing unit (23) produces a CO-containing gas which is returned to the shift conversion stage (10) through line (41). If a CH₄-rich gas is produced simultaneously, it is returned through line (42). In order to support refrigeration, a natural gas stream at a pressure of 10 to 100 bar and preferably not less than 30 bar is supplied through line (1 b). This stream is allowed to expand in the wash plant (23) in order to accomplish a pressure reduction by at least 8 bar, preferably not less than 25 bar. The expanded natural gas may then be extracted, likewise through line (42), for example.

[0014] The wash (23) is controlled such that the synthesis gas accumulating in line (24) already exhibits a molar H₂:N₂ ratio of about 3:1. This synthesis gas is heated up in an indirect heat exchanger (45), compressed in compressor (46) and flows through line (24 a) to an ammonia synthesis unit which includes the indirectly cooled reactor (25) and the adiabatic reactor (26). Recirculated synthesis gas from line (27) together with the fresh synthesis gas from line (24 a) at temperatures in the range of 100-200° C. enters, through line (27 a), reactor (25) from where it flows through the heat transfer tubes (28) or ducts, with the gas acting as a cooling fluid and removing heat from the catalyst bed (25 a). Alternatively, boiling water can be used as a cooling fluid in the ammonia synthesis.

[0015] The synthesis gas leaves the reactor (25) through line (29) at temperatures in the range of 300-500° C., contacting the catalyst bed when arriving in reactor (26). The NH₃ forming reaction is exothermal so that the mixture leaving through line (30) exhibits temperatures of 400-600° C., thus having to be passed through a cooler (31). Subsequently, the NH₃-containing synthesis gas, arriving through line (32), enters reactor (25) and flows through its indirectly cooled catalyst bed. The outlet temperature through line (33) is in the range of 300-500° C. and preferably 380-430° C. The product mixture in line (33) has an NH₃-concentration of not less than 20% (vol.) and additionally contains mainly N₂ and H₂. This mixture is submitted to multistage cooling (34), subsequently passing on to a separator (35) from which raw NH₃ is withdrawn in liquid state through line (36). The gaseous components are extracted through line (27) and returned as recycle gas.

[0016] The raw NH₃ produced can be removed totally or partly through line (37) and routed to state-of-the-art use. Moreover, the raw NH₃ may be totally or partly sent to a state-of-the-art urea synthesis unit through line (38). The urea produced is withdrawn through line (39).

[0017] In the process according to FIG. 2, the synthesis gas coming from the shift conversion stage (10) though line (11) is passed through an indirect cooling stage (12), compressed in the compressor (15) and fed to a CO₂ absorber (14 a) through line (13). In the absorber, CO₂ is removed using a weak carbamate solution which is supplied through line (18) and originates from the urea synthesis unit. Spent CO₂-laden wash solution is drawn from line (16) and fed to the synthesis (21). The partially purified synthesis gas flows through line (22) to the fine screening unit (23 a) which can be designed e.g. as a liquid nitrogen washing system, as a pressure swing adsorption plant or as a catalyst methanation system. Line (1 b) is only suited for the liquid nitrogen wash.

[0018] The ammonia synthesis is operated as described for FIG. 1. The product mixture coming from the cooling stage (34) is routed through line (33 a) to an adsorber (35 a) where NH₃ is washed out from line (50) by means of water. The NH₃-containing water is fed to the urea synthesis stage (21) via line (51); details are described in EP-A-0905 127. As for the rest, the reference figures of FIG. 2 have the same meaning as explained for FIG. 1.

[0019] The process according to the invention has the following main advantages over known processes:

[0020] 1. Steam reforming is not needed, meaning that a large and expensive plant unit can be omitted. Another advantage is that higher pressures can be applied for cracking methane and other hydrocarbons than would be viable with steam reforming.

[0021] 2. Preferably, the nitrogen needed for the H₂-N₂ synthesis gas is only added in the liquid-nitrogen wash and does not have to be carried by the gas through the upstream hydrogen production and purifying stages.

[0022] 3. In the liquid-nitrogen washing stage, methane gas can conveniently also be separated and returned to the autothermal reformer. This allows the reformer to be operated at very low temperatures of about 950° C. without having to ensure that the gas mixture produced in the reformer is methane-free. Furthermore, a natural gas stream supplied at a pressure of 10 to 100 bar may be expanded in the wash unit using liquefied nitrogen, for refrigeration purposes (Joule-Thompson effect).

[0023] 4. In the wash with liquid nitrogen, conveniently a CO-rich gas stream is also produced which is returned to the CO conversion stage. A residual CO content in the converted gas mixture is therefore not detrimental and may amount up to 8% (vol.), mostly not more than 4% (vol.). As a consequence, it is feasible to use sturdy and low-cost iron catalysts for shift conversion so that more sensitive copper catalysts are not needed.

[0024] 5. Gas cleaning by means of a liquid nitrogen wash yields an ultra-pure H₂-N₂ synthesis gas so that bleeding of part of the recycle gas from the NH₃ synthesis can be omitted, totally or for the most part.

[0025] 6. The waste heat produced is sufficient to cover the total energy demand including compression energy for the NH₃ synthesis and subsequent urea synthesis.

[0026] 7. The consumption of natural gas, referred to the net calorific value, is only about 27.3 GJ/t for NH₃ production and not more than about 19 GJ/t for urea production, which is extremely low compared with the known processes. This natural gas consumption has been taken as a basis for the following example.

[0027] 8. The plant for the process can be modularized and can be erected on a relatively small area of land.

EXAMPLE

[0028] The process configuration considered is as depicted in FIG. 1, wherein 3000 t of ammonia or 5263 t of urea may be produced per day. The following data have been partly calculated.

[0029] Through line (1), natural gas and through line (1 a) water vapor are supplied in accordance with a molar water vapor: carbon ratio of 2.55. The data relating to quantities, temperatures, pressures and gas compositions (in % (vol.)) are as listed in Table I: TABLE I Reference figure 1 43 7 11 24a 27a 33 20 Rate (t/h) 92 263 336 357 127 382 382 162 Temperature 25  65  95  32 168 175 403  32 (° C.) Pressure (bar) 55  61  60  57 137 143 140  3 Composition CH₄ 91.3  27.0  1.8  2.0 — — —  0.8 C₂H₆  5.8 — — — — — — — CO —  1.6  10.6  1.1 — — — — CO₂  1.9  0.6  7.1  16.7 — — —  99.0 Ar — —  0.3  0.5 — — —  0.1 H₂ —  3.2  38.7  47.5  74.8  70.8  54.1  0.1 N₂  1.0  0.3  0.4  2.3  25.2  24.4  18.9 — H₂O —  67.3  41.1  29.9 — — — — NH₃ — — — — —  4.8  27.0 —

[0030] The oxygen in line (4) has an O₂ content of 95% (vol.). The synthesis gas in line (24) contains less than 5 ppm (vol.) CO and about 25 ppm (vol.) Ar. The NiO catalyst (3 a) and the catalysts for the NH₃ synthesis are commercial grade (e.g. manufactured by Süd-Chemie, Munich (DE), Type G-90 and AS-4). The reformer (3) is operated at an outlet temperature of 950° C., the level corresponding to the lowest total gas consumption.

[0031] The shift conversion stage (10) comprises first a gas-cooled reactor of similar design to reactor (25), followed by an intermediate cooler and an adiabatic reactor with catalyst bed. The conversion catalyst is a commercial-grade Fe-Cr catalyst (Type G-3C from Süd-Chemie). The residual CO content of the converted gas is only 1.6% (vol.) (calculated dry), the H₂:CO₂ volume ratio being 2.84 (calculated dry).

[0032] For the gas wash units (14, 17), the Rectisol process is applied where CO₂ is removed with methanol at −58° C. In the liquid nitrogen wash (23), the synthesis gas is initially cooled to −185° C. whereby CH₄ is condensed, separated and removed through line (42).

[0033] In contact with liquid N₂, the CO₂ content is condensed, separated and passed through line (41) to the conversion stage. The composition of the stream in lines (41) and (42) is shown in Table II (in % (vol.)). TABLE II (41) (42) CH₄  5.13 52.54 CO 21.18 12.27 CO₂ — 0.53 Ar  7.18 8.64 H₂  9.76 6.75 N₂ 56.75 19.27

[0034] In the cooling system (34), 65% of the NH₃ produced is liquefied by applying cooling water. A part stream (purge gas) is withdrawn to remove impurities from the recycle gas of the NH₃ synthesis unit. 

What we claim is:
 1. A process for the catalytic production of ammonia from a nitrogen/hydrogen mixture wherein natural gas together with O₂-rich gas containing at least 70 vol. % of O₂ is passed to an autothermal reformer where at temperatures in the range of 900-1200° C., a pressure of 40-100 bar and in the presence of a reforming catalyst, raw synthesis gas is produced which, calculated dry, exhibits an H₂ content of 55-75% (vol.), a CO content of 15 to 30% (vol.), a CO₂ content of 5-30% (vol.) and an H₂:CO volume ratio of 1.6-4, the raw synthesis gas being extracted from the autothermal reformer, cooled, passed through a shift conversion stage for converting CO into H₂, the converted synthesis gas being extracted at an H₂ content, calculated dry, of not less than 55% (vol.) and a CO content of not more than 8% (vol.), the converted synthesis gas being then subjected to a multistage gas washing process for removing CO₂, CO and CH₄, where the synthesis gas is contacted with liquid nitrogen in at least one gas washing stage and an N₂-H₂ mixture is produced which is fed to an ammonia synthesis plant for the catalytic production of ammonia.
 2. A process as claimed in claim 1, wherein the ammonia produced in the ammonia synthesis unit is at least partly converted to urea with CO₂.
 3. A process as claimed in claim 1 or 2, wherein CO₂ is removed from the converted synthesis gas in at least one gas washing stage, the removed CO₂ is recovered at least partly and used for urea production.
 4. A process as claimed in claim 1 or one of the following claims, wherein CO₂ is removed from the converted synthesis gas in a physical wash with methanol at temperatures in the range of −70 to −20° C.
 5. A process as claimed in claim 1 or one of the following claims, wherein CO-containing gas is separated from the synthesis gas in a gas washing stage operated with liquid nitrogen and passed on to catalytic conversion.
 6. A process as claimed in claim 1 or one of the following claims, wherein the N₂-H₂ mixture is routed through at least two catalyst-containing reactors in the ammonia synthesis unit where the N₂-H₂ mixture serves as a cooling fluid in a reactor for indirect catalyst cooling.
 7. A process as claimed in claim 1 or one of the following claims, wherein the synthesis gas leaving the shift conversion stage exhibits an H₂:CO₂ volume ratio of 2.5-3.0 (calculated dry).
 8. A process as claimed in claim 1 or one of the following claims, wherein a natural gas stream at a pressure of 10 to 100 bar is routed to the gas wash unit operated with liquefied nitrogen in which the pressure of the said stream is reduced by at least 8 bar. 